Unrefined and partially refined oil may comprise nickel that can be difficult to remove. In particular, oils and fats that have been hydrogenated with the use of a nickel-containing catalyst commonly still contain, after removal of the catalyst by filtration, a substantial amount of nickel. The nickel content of such filtered hydrogenated oils and fats may be as high as 50 or 100 ppm. These residual traces of nickel occur in the form of soap and/or as colloidal metal. For various reasons, e.g. to prevent oxidation, it is desirable for the nickel content of oils to be low, e.g. below 1 ppm. This is especially the case for oils to be used in edible products, in which oils the nickel content should preferably not exceed 0.1 ppm.
A variety of processes has been proposed as post-refining treatment of hydrogenated oil, in particular to remove residual nickel from filtered hydrogenated oil or to prevent the occurrence in oil of objectionable quantities thereof.
In U.S. Pat. No. 2,365,045 it is advised to add activated carbon to the oil before, during or after hydrogenation, but preferably before the hydrogenation. The free fatty acid content of the oil to be thus treated should preferably be not more than 0.05%. It is therefore considered to be desirable to employ as oil to be hydrogenated, an alkali-refined oil. After the hydrogenation, the mixture comprising the oil, the catalyst and the carbon, which may or may not have been present during the hydrogenation, is filtered to recover the hydrogenated oil. The addition of the sorptive material serves to facilitate the removal of colloidally suspended nickel particles and to absorb soaps.
According to U.S. Pat. No. 2,602,807, the removal of nickel catalyst from hydrogenated oil by incorporating bleaching clay in the oil and filtering the mixture obtained, can be improved by employing acid-activated clay. The clay may be added to the oil/catalyst slurry or it may be added to the oil from which the major part of the catalyst has already been removed by filtration. The process can, for example, be carried out by, prior to filtration, first adding a small amount of concentrated phosphoric acid or sulphuric acid to ordinary bleaching clay and then adding the thus acidified clay to the oil or by adding both acid and bleaching clay to the oil.
U.S. Pat. No. 2,650,931 advises, in order to remove residual metal contaminants from filtered hydrogenated oil, to intimately mix the oil with an aqueous solution of an acid in which the metallic salts are soluble, and to subject the resulting mixture to a centrifuging operation in which the aqueous acid solution is centrifugally separated from the cleaned oil. Suitably a diluted aqueous solution of, for example, citric acid, phosphoric acid or tartaric acid is employed in an amount of about 10% of the amount of oil.
In U.S. Pat. No. 2,654,766 a number of treatments are proposed to obtain suitable hydrogenation results:
The hydrogenation is carried out in the presence of lignin, which is said to allow better separation of nickel traces in the filtered hardened fat due to the inactivation of nickel soaps which are said to be usually formed in the course of hydrogenation using a nickel catalyst.
According to the specification, the formation of nickel soap can be substantially reduced by subjecting the oil to a pretreatment wherein the oil is heated to above 100.degree. C. under hydrogen pressure in the presence of a small amount of spent metal catalyst, e.g., spent nickel catalyst. Improved results are said to be obtainable by carrying out this heat pretreatment in the presence of a small amount of activated bleaching earth and preferably also of activated carbon and filtercel.
The thus pretreated and filtered oil may subsequently, prior to hydrogenation, be subjected to treatment with phosphoric acid and/or sodium phosphate so as to separate metal soaps. This treatment is carried out by heating the oil with a diluted aqueous acidic solution, allowing the mixture to settle and removing the aqueous soap-containing sludge. Subsequently, the oil may be filtered with a small amount of filtercel, or, alternatively, to remove free acidity, the oil may be sprayed with a caustic soda solution, followed by repeated washing with hot water to remove traces of soap.
A similar treatment with a diluted solution of phosphoric acid and sodium phosphate may be applied after the hydrogenation to remove, e.g., nickel soaps. This treatment may be followed by a treatment with about 0.01-0.02% organic acid such as oxalic acid, citric acid or acetic acid to remove iron traces. Subsequently, for example activated clay can be added to effect bleaching of the oil and/or the oil can be deodorised, to complete the refining procedure.
In U.S. Pat. No. 2,783,260, a process is described for removing hydrogenation odours and flavours from fatty oils, comprising adding about 0.5-4% concentrated phosphoric acid to the hydrogenated oil, maintaining the mixture with agitation at about 60.degree.-95.degree. C. while injecting air therein, subsequently adding a neutral bleaching earth and maintaining the resulting mixture at about 130.degree.-140.degree. C. and then cooling and filtering the mixture. The oil to be thus treated should not have a free fatty acid content higher than 0.1%. Oil having a free fatty acid content above 0.1% should first be subjected to alkali refining, before applying the treatment with phosphoric acid, air and bleaching earth.
In Fish Oils, edited by M. E. Stansby, published by the Avi Publishing Company, Inc. in 1967, p. 403, it is described how to subject filtered, hydrogenated fish oil to a post-refining process comprising treating the oil with 0.1N caustic soda solution, washing it three times with hot water, and then vacuum-drying it at 95.degree. C. Subsequently, the oil is bleached with activated earth and deodorised by injection of superheated steam under reduced pressure.
Similarly, Oleagineux, 28 N.degree. 7, (1973), pp. 356-359, describes the treatment of crude hardened oil, after filtering it, with a dilute alkali wash, followed by a hot water wash, and then by drying of the oil, addition of earth, filtering and deodorising. If continuous centrifugal equipment is employed, the hot water wash step may be omitted. Alternatively, it is said that, especially in the case of hardened vegetable oils, the alkali neutralisation may be omitted and the oil may be post-refined by merely adding a small amount of activated earth before filtering a second time, and then stripping the oil to cause deodorisation and removal of free fatty acids. To protect the oil against oxidation, it is suggested to add metal sequestrants such as citric acid at any convenient time after filtering and suitably at the beginning of the deodorisation.
In GB No. 1,531,203 it is stated that filtered, hydrogenated organic liquids, e.g., oils, often contain residual amounts of metal hydrogenation catalysts which must be removed by subsequent steps which are usually termed "post-bleaching", where the residual traces of the metal catalyst are removed through the use of neutral scavengers of compounds capable of forming inactive complexes with the metal component. These materials include certain acids such as phosphoric acid and organic acids such as citric acid and tartaric acid. The post-bleaching treatment requires additional filtration with addition of e.g. Filteraid.RTM.. As an alternative way of post-refining hydrogenated oil, GB No. 1,531,203 teaches to subject hydrogenated oil in admixture with a finely divided dispersed solid absorbent, in the absence of oxygen, to electrofiltration. Alternatively, the absorbent may be admixed prior to the hydrogenation reaction.
In DOS No. 2,854,949 it is proposed to remove nickel particles from hydrogenated oil by passing the oil through a magnetic field.
According to Bailey's Industrial Oil and Fat Products (Volume 2, fourth edition, John Wiley & Sons, p. 37 (1982)), removal of residual nickel, occurring in filtered hydrogenated oil in the form of soap or colloidal metal, is usually accomplished by a so-called post-bleaching step, in which the filtered oil is treated with 0.1-0.2% of bleaching earth at about 180.degree. F. (82.2.degree. C.) and filtered. A very small amount of phosphoric acid or other metal scavenger is sometimes added in the bleaching step. It is stated that, alternatively, activated carbon can be used, which is added to the oil along with the catalyst prior to hydrogenation. Carbon in an amount equal to 10-20 times the amount of nickel in the catalyst is reported to yield a metal-free filtered oil.
Thus, a large variety of processes has been proposed to achieve an acceptably low residual nickel content in hydrogenated oil. However, various disadvantages are attached to these processes. Despite all the attention devoted to solving this problem, no entirely satisfactory solution has been found.
Firstly, not all proposed processes have the desired effect of substantially reducing the residual nickel content or preventing the occurrence of an objectionably high nickel content. For example, the effect on the nickel content of filtered hydrogenated oil of washing the oil with diluted aqueous solutions, is limited.
Processes wherein an absorbent is admixed with the oil/catalyst slurry, whether added before, during or after the hydrogenation, prior to the filtering, have the disadvantage that the catalyst that is retrieved from the filtration is diluted with the spent absorbent. This affects the possibilities of re-using the catalyst in subsequent hydrogenations. Moreover, the increasing quantities of catalyst-absorbent mixture that need to be employed when re-using the mixture repeatedly, cause a corresponding increase of oil loss at the filtration.
Several of the above-described processes employ diluted aqueous solutions, in particular diluted aqueous acidic solutions. In these processes the oil is subjected to a so-called washing treatment, i.e. a relatively large amount of the aqueous solution, e.g. about 10 wt. % calculated on the oil, is admixed with the oil. The mixture may then, for example, be given a residence time or be heated. Subsequently, an aqueous phase containing contaminants is separated off and refined oil is recovered by means of gravitational force, e.g. by centrifugal separation or by draining the aqueous contaminants-containing phase from the bottom of the vessel. Such treatment is often followed by one or more washing steps with hot water.
Apart from the fact that such processes often do not adequately reduce the nickel content, it is a major disadvantage that these treatments produce large quantities of effluent.
A further group of processes that can be distinguished in the prior art consists of the ones in which the oil is treated with chemical reagents, other than adsorbents, but in which no large amounts of aqueous solutions are employed. These processes do not have the disadvantage of producing large volumes of aqueous effluent. As described above, in these processes concentrated acids, e.g. phosphoric acid, citric acid or sulphuric acid, are employed, and the chemical substance added to the oil contains only a small proportion of water or practically no water at all. When using such substances, there is a substantial risk of corrosion. Consequently, corrosion-resistant equipment, e.g. stainless steel equipment, must be used for handling these substances. Moreover, when using such aggressive chemical reagents, the risk of undesired side reactions occurring, e.g. hydrolysis of the oil, is substantial.
Some of the processes described above are very expensive. For example, the required investments for installing a magnet or an electrofilter with factory-scale capacity are very high.